A Basic but Tricky (?) Problem about Shock

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A Basic but Tricky (?) Problem about Shock

Postby henriette on March 7th, 2017, 4:32 am 

Dear,


Let's consider you are at a fixed position in the lab and you drop vertically a box on a moving conveyor belt. Show that the box tends to rotate backward.


This problem seems trivial at first glance, maybe it is, but it turns tricky : How should the discontinuity of the linear momentum of the dropped box at the time of the shock be considered ?

I envisioned that the reaction force of the conveyor on the box must be located somewhere ahead of the mass center of the box, thus yielding an angular momentum that causes the rotation of the box, but alas the description of the discontinuity of the linear momentum does not provide any insight about that...
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Re: A Basic but Tricky (?) Problem about Shock

Postby zetreque on March 7th, 2017, 10:02 am 

Not sure I understand what you are saying but a ball would always rotate backwards. If the box hits on the back edge first then it could possibly rotate forward and depends on the friction coefficient.
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Re: A Basic but Tricky (?) Problem about Shock

Postby Dave_Oblad on March 7th, 2017, 8:14 pm 

Hi all,

Just jump down on a Conveyor Belt yourself. Your center of mass is halfway between your head and feet. Your feet, having friction now, wants to move in the direction of the conveyor belt. Your center of Mass wants to stay put. Thus, you must have your Feet pulled out from under you and you land on your Back. Assuming, of course, that your orientation is facing the same direction as the motion of the belt. Otherwise, given a 180 rotation of the way you are facing.. you land on your Front.

However, this rotation force depends on the Center of Gravity, so a ball will roll but a sheet of paper, landing flat, will hardly rotate at all.

Regards,
Dave :^)
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Re: A Basic but Tricky (?) Problem about Shock

Postby someguy1 on March 7th, 2017, 8:55 pm 

Friction, right? If the conveyor belt is frictionless the box will land and stay at the same point in space as the belt moves under it.
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Re: A Basic but Tricky (?) Problem about Shock

Postby Braininvat on March 7th, 2017, 9:11 pm 

A frictionless conveyor belt? That would seem like a truly terrible design feature.

LOL
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Re: A Basic but Tricky (?) Problem about Shock

Postby Dave_Oblad on March 8th, 2017, 2:33 pm 

Hi Biv,

That's so funny.. I almost cried laughing so hard. I'm still laughing and can't see my keyboard to type this.

Best regards,
Dave :^)
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